1. Field of the Invention
The present invention relates to a magnetic resonance system.
2. Description of the Prior Art
Magnetic resonance systems are known that have a first installation unit in which a control processing device is arranged, as well as a radio-frequency coil arrangement for transmitting and receiving magnetic resonance signals, the radio-frequency coil arrangement being controlled or read out by a transmit device. The transmit device includes a digital transmission unit built into the first installation unit for emitting a digital transmit signal to an analog transmit unit, which outputs an analog transmit signal based on the digital transmit signal. A reception device has at least one analog reception unit for conversion of an analog received signal into a digital received signal and at least one digital reception unit for digital demodulation of the received signal.
Radio-frequency coils in magnetic resonance systems are used both for radio-frequency excitation of spin in a target region to be recorded and also to receive the magnetic resonance signals from this target region. To transmit the corresponding sequences in such cases, the entire radio-frequency coil arrangement is activated, but for receiving, to achieve a better resolution for example, only parts of the arrangement are used in each case, so that a large number of reception channels is produced. To implement the functionalities of the radio-frequency antenna arrangement, suitable transmit or reception electronics are needed, i.e. a transmit device and a reception device. To comply with the coherence conditions for magnetic resonance images, however, a precise synchronization and tuning between the transmit device and the reception device is necessary in such cases.
Both the transmit and the reception device can be subdivided in this case into an analog subunit and a digital subunit. In this case a low-frequency signal describing the desired sequence is digitally modulated or mixed in the digital transmit unit. The digital transmit unit communicates with the analog transmit unit, in which the digital transmit signal is converted by means of a Digital-Analog converter (D/A converter) into an analog transmit signal. It can then be modulated accordingly into an analog signal. This type of analog modulation, with the digital transmit signal initially lying on an intermediate frequency (ZF) and then being modulated onto the target frequency, is not absolutely necessary. The signal can also be processed completely digitally. In such a case the analog transmit unit only includes the D/A converter. The analog transmit signal is then sent out via the radio-frequency coil arrangement communicating with the analog transmit unit.
For reception of signals the received signals are initially directed to the analog reception unit where they are correspondingly demodulated if necessary, and converted by an A/D converter into a digital received signal. In the case of purely digital control the analog reception unit also only includes the A/D converter. Using corresponding communication links the digital reception signal is forwarded to a digital reception unit for digital demodulation. From there it is forwarded into a data preparation unit, in which the demodulated digital received signal is converted into data usable for image generation and correspondingly filtered. An image processing device then processes this data into an image.
As mentioned, it is essential in this case to obtain the necessary coherence condition. Usually both the digital transmit unit and the digital reception unit are built into a control processing unit used for controlling the operation of the magnetic resonance system, with the two units being jointly controlled by a suitable digital frequency generator unit, for example a numerically controlled oscillator (NCO).
The number of reception channels has a decisive influence on the quality of the magnetic resonance images recorded, especially on the signal-to-noise ratio in the recorded images. Known magnetic resonance systems have a limited number of such reception channels. There are essentially two reasons for this. One is that it is not possible to expand the analog reception units simply as required. Another is that digital reception units or data preparation units can process only a specific number of channels, which is usually not the same as the number of channels processed by an analog reception unit, so that a number of digital reception units are required for each analog reception unit, or vice versa. The connection options for a control processing device are, however, limited both by space and also by the number of the available connection options or the plurality of the devices needed for control. Such control processing devices or their components respectively are in such cases mostly arranged in an installation unit, for example a rack or electronics cabinet, in which the components can be plugged and provided with cables to interconnect them. These typical slots are mostly all occupied for a control processing device. Another factor is that, because a frequency generator unit is needed, the digital transmit unit and the digital reception unit are often integrated into one unit, but only a digital transmit unit is needed. In the final analysis the number of reception channels and thereby the signal-to-noise ratio are thus restricted.
A magnetic resonance microscope is known from United States Patent Application Publication No. 2002/0030491, in which a number of channels is created by recording a number of samples in parallel in different chambers shielded from one another with their own gradient coils and their own radio-frequency coils in a single large basic field.
A use of a number of transmit channels is known from PCT Application WO 2004/061469 A1 to enable the radio-frequency field to be better adapted.
A determination of an ideal analog synthesizer frequency is known from DE 198 44 420 C1.